1. Field of the Invention
The present invention relates to a color signal generating circuit and a color signal synthesizing circuit.
2. Description of the Background Art
In a digital camera, a taken image is displayed on a monitor and textual information such as the date, time and counter of photo-taking is overlaid on the image. Predetermined buttons operation allows various menus to be shown on the camera image. The data thus displayed on camera image in an overlaid manner is referred to as OSD (On Screen Display) data.
An image such as textual information overlaid on the taken image is displayed in multi colors these days. This makes it possible to change colors depending on the contents of information to be displayed or to perform gradation display, thereby improving GUI (Graphic User Interface).
As for the pixels to be concerned, overlaid video images can be displayed with transparency by mixing colors for displaying textual information and colors for displaying the taken image at a predetermined mixture ratio. This achieves not only multi-color display but also various changes made in the display on the monitor.
However, there is a problem that using various colors for overlaid display requires a large storage capacity to store the color information. This problem is particularly serious in electronic appliances such as digital cameras and digital movies which have been increasingly reduced in size and cost.
FIGS. 7 to 9 are circuit diagrams of color signal synthesizing circuits which has been conventionally used. FIG. 7 shows a color signal synthesizing circuit capable of performing overlaid display with 16 colors, and FIGS. 8 and 9 show a color signal synthesizing circuit capable of performing overlaid display with 256 colors. The flows of the processes of these circuits and the details of the circuits will not be described here since they are equal to the description in embodiments of the present invention.
In the case of 16 colors-equipped circuit (FIG. 7) using 4-bits data per pixel in overlaid display, a color storage unit 141 holds 16 color registers (color registers 0 to 15). On the other hand, in the case of 256 colors-equipped circuit (FIG. 8) using 8-bits data per pixel in overlaid display, a color storage unit 151 holds 256 color registers (color registers 0 to 255). This brings about the problem that a growing number of colors expands the circuit scale, thereby increasing the cost and power consumption.
FIG. 9 shows a case adopting an SRAM as a color storage unit 161. This case can have a circuit scale smaller than the case with 256 color registers as shown in FIG. 8, but has the problem of cost increase.